1. Field of Invention
This invention relates to wavelength selection devices and more particularly to an improved band pass interferometer, which, among other things, has a defined passband, has polarization insensitivity, has low insertion loss, is reliable and is cost effective. In at least one of several preferred embodiments, the improved interferometer is also tunable.
2. Brief Description of the Prior Art
Optical communications system have been proliferating rapidly because of their high bandwidth, made available by the carrier frequency, in the hundreds of THz range, which corresponds to (0.9 . . . 1.6) μm wavelength range. (Throughout the description of this invention, either wavelength or frequency will be interchangeably used, depending on the context.) Optical fibers and optical amplifiers have a passband of at least 30 nm, which allow simultaneous work of many carriers having different wavelengths. The problem in this case is to generate each carrier with its specific wavelength at the source side and to select only one carrier (wavelength) at a time per communication channel at the receiver side. A key element of the communication channel is a filter with a narrow passband, low attenuation in the passband, sharp edges and high attenuation in the rejection or stop bands. Fixed-frequency filters with such characteristics exist, but their use in DWDM systems also require the use of multiplexers to combine multiple wavelengths (channels) into the same fiber and demultiplexers to extract the individual wavelengths from a multitude of wavelengths carried by an optical fiber. The multiplexer/demultiplexer introduces additional power losses and adds overall complexity to the system. A tunable filter could simplify wavelength selection in a communication system and could give flexibility by allowing fast and random frequency selection per channel.
Fabry-Perot interferometers are widely used as narrow-band, tunable elements in tunable filters. The basic configuration of a conventional Fabry-Perot interferometer is schematically shown in FIG. 1 and is also disclosed in the well-known publication, M. Born, E. Wolf, “Principles of Optics”, Chapter 7.6, pp. 359–409, 7-th Edition, Cambridge University Press, Cambridge, 1999. The Fabry-Perot interferometer consists basically of two transparent plates (substrates) with parallel and flat surfaces 101 and 102 facing each other, one surface covered with a highly reflective layer 103 having a reflection coefficient r1, and the other surface covered with a highly reflective layer 104 having a reflection coefficient r2. An incoming beam 105, wide enough to cover the aperture of the two reflective layers 103 and 104, and propagating within a medium having a refractive index n1, (which is usually the air), is incident on the plate 101 under the incidence angle θ. Fabry-Perot interferometers usually have highly reflective layers r1≈r2 and small, but not zero incidence angle θ≈0. The reflective layer 103 splits the incident beam 105 into the reflected beam 107 and the transmitted beam 106, which will propagate in the medium 108 with a refractive index n2. The optical medium 108 completely fills the gap between the reflective layers 103 and 104. Only the rays transmitted through the reflective layer 103 are important for the transmission properties of the Fabry-Perot interferometer and will be further considered. The reflected beam 107 will also be ignored. Each ray of the transmitted beam 106 propagating in the medium 108, bounces many times between the reflective layers 103 and 104. FIG. 1 shows only one ray of the beam 105, without restricting the generality. This ray hits the layer 103 in A1, A2, A3, A4, A5, . . . and the layer 104 in B1, B2, B3, B4, B5, . . . For a small incidence angle θ≈0, there is an infinite number of reflections of the transmitted beam 106 on the reflective layers 103 and 104. All the parallel rays transmitted through the layer 104 belong to the transmitted beam 109, which covers a large area of the reflective layer 104. The transmitted beam 109 contains parallel rays coming from the incident beam 105 and also from a different number of reflections between the reflective layers 103 and 104, from zero, which is the direct transmission, to a very large number, which is the finesse of the interferometer. All of these rays interfere between them and generate fringes in the far field. The fringes are parallel and equidistant lines, all of them being also parallel with the straight line generated by the intersection between the incident beam 105 and the first reflective layer 103. A lens 110 focuses the fringes on a screen 111, up to a single line 112. For a given wavelength λ, the resulting field in 112 produces either a maximum or a minimum of interference depending on the incidence angle θ, the spacing d between the reflective layers 103 and 104, and of the refractive index n2 of the media 108. The contribution of the refractive index and of the thickness of the plates 101 and 102 are ignored for the purpose of this explanation, because they are constant. For a constant wavelength λ, the beam intensity on the line 112 can be controlled either by changing the spacing d or by changing the refractive index n2 of the media between the plates 101 and 102, but this case does not have any interest for the present patent. If the intensity of the incident beam is denoted I1 and the intensity of the resulting beam 112 is denoted I2, then the transmission function of the Fabry-Perot interferometer is denoted T=I2/I1. For constant values of the incidence angle θ, of the plate spacing d and of the refractive index n2, the transmission T is a function of wavelength T(λ) and the Fabry-Perot interferometer has wavelength selective properties. Its wavelength selectivity increases with the number of reflections on the reflective layers 103 and 104, parameter usually expressed as finesse. Higher values of r1 and r2 (higher finesse) means better wavelength selection, but these values also decrease the maximum transmission. At normal incidence θ=0, the Fabry-Perot interferometer becomes a resonant cavity with eigenmodes and the transmission function T(λ) has only discrete values. This case was very well analyzed by A. G. Fox and T. Li, in “Resonant Modes in a Maser Interferometer” published in “The Bell System Technical Journal”, pp. 453–488, March 1961 and G. D. Boyd and K. Kogelnik, in “Generalized Confocal Resonator Theory”, published in “The Bell System Technical Journal”, pp. 1347–1369, July 1962. A continuous tunability T(λ) of the Fabry-Perot interferometer cannot be achieved for a wide wavelength range.
Many patents disclose Fabry-Perot interferometers of various forms. For example, U.S. Pat. No. 5,361,155 discloses a tunable filter realized by changing the incidence angle into the Fabry-Perot interferometer. The input beam comes from an input monomode optical fiber and is collimated by a lens, onto a Fabry-Perot interferometer. A lens focuses the output beam from the interferometer onto an output monomode optical fiber. There is a unique relation between the incidence angle into the Fabry-Perot interferometer and the center wavelength of the transmission peak. A rotating plate with flat-parallel faces compensates the output beam displacement, to keep the beam in the pupil of the receiving optical fiber. The main drawback of this approach is that the bandwidth depends on the incidence angle to Fabry-Perot filter, i.e. the bandwidth is not constant in the tuning range of the filter.
In the U.S. Pat. No. 5,917,626, the light beam is focused on the interference filter using a GRIN lens. Changing the incidence point to the entrance into the GRIN lens changes the angle of incidence to an interference filter. The incidence angle is adjusted by rotating the plate or substrate into a collimated light beam. The approach of selecting the wavelength by changing the incidence angle to an interference filter has the advantage of tunability over the whole wavelength range required by DWDM applications, but has also two major drawbacks: variable bandpass in the working range and polarization dependence, especially when the incidence angle is greater than few degrees.
U.S. Pat. No. 5,710,655 claims the use of the dependence of the refractive index of a liquid crystal to the applied voltage to tune the interferometer. The gap of a Fabry-Perot interferometer contains a voltage-controlled liquid crystal cell. The refractive index of the liquid crystal is voltage-dependent, which makes the interferometer tunable into a certain range. For those knowledgeable in the field, it is obvious that the patent ignores the changes in the beam polarization induced by the liquid crystal, which affects the interference of the output beams and makes the device polarization-sensitive.
U.S. Pat. No. 5,073,004 uses a Fabry-Perot interferometer built at the ends of two ferrules having the mirrors embedded in their volume. Wavelength selection is done by adjusting the gap between the mirrors. However, comments about the tuning range, transmission and bandwidth in the passband, and diffraction losses are not disclosed and accordingly are left up to speculation.
U.S. Pat. No. 5,739,945 shows a low-order, integrated Fabry-Perot interferometer used as the main element of an optical filter. The transfer function has many peaks in the working range of the filter, but there is no mention about the free spectral range (FSR), the transmission value and the bandwidth in the passband.
U.S. Pat. No. 4,976,513 describes another integrated Fabry-Perot interferometer, having distributed feedback Bragg (DFB) reflectors as mirrors. The frequency of the cavity is adjusted by the injection current into a phase-control section, located between the DFB reflectors. Because of the geometry built into a waveguide, the Fabry-Perot interferometer is very low-order and has a large FSR. While the center wavelength range is acceptable for DWDM applications, the slopes of the transfer functions are unfortunately very mild.
U.S. Pat. No. 5,212,584 discloses a Fabry-Perot interferometer as a wavelength selective device. Here, the interferometer is built into the volume of a transparent plate with plane-parallel surfaces, covered with reflective multiple coatings. The author claims wavelength selection by changing the incidence angle on the plate. Wavelength locking is achieved by changing the refractive index of the plate. The author considers that the changes to the plate thickness with temperature produce negligible effects on tunability, when compared with the changes of the refractive index induced by temperature. The tuning range and the passband reported are acceptable for DWDM applications. However, the limitation with this approach comes from the tuning speed.
Other approaches to implementing tunable optical filters are disclosed in several publications. For example, Fabry-Perot tunable filters with plane mirrors made in microelectromechanical (MEM) technology are disclosed in the papers N. Chitica et. al., “Monolithic InP-based Tunable Filter with 10-nm bandwidth for Optical data Interconnects in the 1550-nm Band”, IEEE Photonics Technology Letters, vol. 11, No. 5, pp. 584–586, May 1999; and A. Spisser et. al., “Highly Selective and Widely Tunable 1.55-μm InP/Air-Gap Micromachined Fabry-Perot Filter for Optical Communications”, IEEE Photonics Technology Letters”, vol. 10, pp. 1259–1261, September 1998. However, these filters have some important drawbacks: low tuning range, wide passband because of their low-order mode of operation, high diffraction losses because of the poor optical quality of the mirrors or Bragg reflectors and difficulties in connecting with optical fibers.
In another implementation of a tunable Fabry-Perot resonators using MEM technology, as disclosed in P. Tayebatti, et al., “Microelectromechanical Tunable Filter With Stable Half Symmetric Cavity”, Electronics Letters, vol. 34 (1998), No. 20, pp. 1967–1968, the plane mirror of the cavity is fixed on the substrate and the spherical mirror of the cavity moves on top of it driven by a control voltage, to tune the interferometer. The filter has a single transmission peak about 0.8 nm wide at −30 dB in the whole working range of 40 nm. An input optical fiber coupled with a lens sends the beam into the Fabry-Perot interferometer and the output beam from the interferometer is collected by an output fiber. The collimated beam at the entrance of the Fabry-Perot interferometer minimizes the high-order modes of the cavity. The major drawback using MEM technology to build the Fabry-Perot interferometer is the open loop operation, which makes impossible an accurate control of the displacement and of the position of the spherical mirror, and implicitly a bad control of the transmission characteristic.
D. Vakhshoori, et al., “2 mW CW Singlemode Operation of a Tunable 1550 nm Vertical Cavity Surface Emitting Laser With 50 nm Tuning Range”, appeared in “Electronics Letters”, vol. 35 (1999), No. 11, pp. 900–901, describe another application of the voltage tunable Fabry-Perot interferometer in MEM technology, for tuning a Vertical Cavity Surface Emitting Laser (VCSEL). The tunable VCSEL has the same drawbacks as the tunable filter described previously. Though the half symmetric cavity has the maximum mechanical stability among all the configuration of Fabry-Perot interferometers, this advantage cannot compensate the disadvantages of the open-loop operation of MEM technology.
In other publications, very low-order Fabry-Perot interferometers having the gap filled with liquid crystals are disclosed. See, K. Hirabayashi et al. “Tunable Liquid-Crystal Fabry-Perot Interference Filter for Wavelength-Division Multiplexing Communication Systems”, published in “Journal of Lightwave Technology”, vol. 11, No. 12, pp. 2033–2043, December 1993. This approach has some major drawbacks: mild slopes between the bandpass and the rejection bands when covering the spectral range for DWDM applications and strong polarization dependence.
Other known methods used to implement a tunable filter, include: Bragg diffraction grating generated by an ultrasound wave, Mach-Zehnder interferometer, fiber-optic interferometer with Bragg reflectors, wavelength-selective properties of some cascaded elements such as a polarizer, an electro-optical birefringent element and an analyzer. While each have their own desirable characteristics, none provide for the required insertion loss in the passband, attenuation in the rejection bands, or the required slope of the transfer characteristic between the transmission band and the rejection bands.
Accordingly, there is a need for a solution that addresses the problems of the prior art.